Reflection

• Regular reflection occurs when parallel light rays strike a smooth surface and reflect in the same direction.

• Diffuse reflection occurs when light rays reflect off a rough surface and are scattered in different directions.

Reflection• The Law of Reflection: When light reflects

off a surface, the angle of incidence is always equal to the angle of reflection.

• Where are plane mirrors used?

Plane mirrorsA virtual image is any image formed by rays that do not actually pass through the location of the image. Light rays are not coming from where the image appears to be.

Curved Mirrors• Concave and convex

• Curved mirrors obey the law of reflection.

Concave vs. Convex mirrorsWhen rays that are parallel to the principal axis strike a concave mirror, they are ALL reflected through the same point, called the focal point.

If several groups of parallel rays are reflected by a concave mirror, each group converges at a point.

Concave vs. Convex mirrors• Concave mirrors are also called converging mirrors

• Concave mirrors are designed to collect light and bring it to a single point

• Examples: Cosmetic mirrors to produce an enlarged image, Telescopes to collect light from a distance source and focus it for viewing

• Concave mirrors can also be used to send out beams of light rays

• Examples: flashlights, car headlights, dental examination lights

Concave vs. Convex mirrors

Concave vs. Convex mirrors• Convex mirrors are also called diverging mirrors

• Convex mirrors spread out rays

• They allow you to view a large region that you could not see with a plane mirror of the same size

• Examples: Store security, parking lot safety, side-view mirrors, camera phones

Curved mirror definitions• When parallel light rays strike a curved mirror, the reflected rays

eventually meet at a common point. The point where the light rays meet, or appear to meet, is called the focal point, F.

• The middle of a curved mirror is the vertex. The principal axis is an imaginary line draw through the vertex.

• The distance from the vertex to the focal point is the focal length, f. • The center of curvature, C, is the center of the circle that would be

formed if you extended the curve of the mirror.

The focal point is half way between the center of curvature and the vertex.

Finding the imageEach time you find the image, you must determine the following:

•Size: Is it smaller, bigger, or the same size as the object?

•Attitude: Is it upright or inverted?

•Location: Is it behind or in front of the mirror?

•Type: Is it real of virtual?

Real vs. Virtual• A real image is formed by light rays that converge at the

location of the image. • A real image can be projected onto a screen: If you place a

piece of paper at the spot where a real image forms, a focused image will appear on the piece of paper.

• A virtual image is any image formed by rays that do not actually pass through the location of the image.

• Light rays are not coming from where the image appears to be.

• Unlike a real image, a virtual image cannot be projected onto a screen.

Drawing a CONCAVE MIRROR ray diagram(Converging mirror)Note: Show real rays using solid lines and virtual rays using dashed-lines.

•Ray 1: parallel from a point on the object to the principal axis (Rays that are parallel to the principal axis will reflect through the focal point on a concave mirror.)

Ray 2: from a point on the object through the focal point. (Rays that pass through the focal point on a concave mirror will be reflected back parallel to the principal axis.)

Ray 3: Draw a line from the same point on the object through the center of curvature. When the ray hits the mirror, it will reflect back on itself.

Draw the image where the rays intersect.

Size: Smaller

Attitude: Inverted

Location: In front

Type: Real

Complete the following tableLOCATION OF OBJECT SIZE OF IMAGE ATTITUDE OF

IMAGELOCATION OF

IMAGETYPE OF IMAGE

More than two focal lengths away from

mirror 

Smaller Inverted In front Real

Between one and two focal lengths away

from the mirror

       

Object is at the focal point

  

       

Object is between the mirror and the focal

point 

       

Drawing a Concave Mirror Ray Diagram Summary

• Ray 1: parallel from a point on the object to the principal axis (Rays that are parallel to the principal axis will reflect through the focal point on a concave mirror.)

• Ray 2: from a point on the object through the focal point. (Rays that pass through the focal point on a concave mirror will be reflected back parallel to the principal axis.)

• Ray 3: Draw a line from the same point on the object through the center of curvature. When the ray hits the mirror, it will reflect back on itself.

•Draw the image where the rays intersect.

Size: LargerAttitude: InvertedLocation: In frontType: Real

Object is between one and two focal lengths

Object is at the focal pointYou cannot draw a ray from the object through the focal point. Therefore, we draw an additional ray to confirm that no image is formed.

No Image Formed

Size: LargerAttitude: Upright Location: BehindType: Virtual

Object is between mirror and focal point

Magnification• One use of concave mirrors is

magnification.

• Magnification: How large or small an image is compared to with the object.

Using the formula• Distances are positive if they are in front of the mirror,

and negative if they are behind the mirror.• Positive heights are above the principle axis, and

negative heights are below the principle axis.• Positive magnifications are above the principle axis, and

negative magnifications are below the principle axis

• If the image is bigger than the object, then the magnification will be greater than 1.

• If the image is smaller than the object, then the magnification will be less than 1.

A concave mirror produces an image on a wall that is 30.0 cm high from an object that is 6.5 cm high. What is the magnification of the mirror?

Sample Solution

Given:

Required:

Solution:

The magnification of the mirror is 4.6 cm.

A person standing 3.00 m from a glass window sees her virtual image 3.00 m on the other side. What is the magnification of the window?

Sample Solution

Given:

Required:

Solution:

The magnification of the window is 1.00 m.

Convex mirrors (Diverging)

• Instead of collecting light rays, a convex mirror spreads out the rays.

Drawing a convex ray diagram:

Ray 1: from a point on the ray parallel to the principal axis (Any ray that is parallel to the principal axis will appear to have originated from the focal point.)

Ray 2: from a point on the object toward the focal point (Any ray that is directed at the focal point on a diverging mirror will be reflected back parallel to the principal axis.)

Ray 3: from a point on the object toward the center of curvature (Any ray that is directed at the center of curvature on a diverging mirror will be reflected back along its own path.)

Draw the virtual image where the rays appear to intersect.

Ray 1: from a point on the ray parallel to the principal axis (Any ray that is parallel to the principal axis will appear to have originated from the focal point.)

Ray 2: from a point on the object toward the focal point (Any ray that is directed at the focal point on a diverging mirror will be reflected back parallel to the principal axis.)

Ray 3: from a point on the object toward the center of curvature (Any ray that is directed at the center of curvature on a diverging mirror will be reflected back along its own path.)

Draw the virtual image where the rays appear to intersect.

Mirror Equation• Rules:• Concave mirrors have positive focal length and convex

mirrors have negative focal length• Distances are positive if they are in front of the mirror,

and negative if they are behind the mirror.• All measurements are made along the principal axis from

the surface of the mirror.

A converging mirror has a focal length of 20 cm. An object is placed 40 cm from the mirror. Determine the image distance.Sample Solution

Given: Solution:

Required:

The image is 40 cm from the mirror.

References• Pearson Investigating Science 10• http://www.blueskiesandyellowdogs.com/2010/08/17/what-i-love-to-see-most-in-my-rear-view-mirror/• http://www.sayulitalife.com/dentalsayulita• http://www.math.ubc.ca/~cass/courses/m309-01a/chu/Fundamentals/reflection.htm• http://www.bnl.gov/slc/DiscoveryActivitiesSLC/MirrorMagic.asp• http://www.bnl.gov/bnlweb/schoolprograms.html• http://shiratdevorah.blogspot.ca/2010/10/rainbow-covenant.html• http://epod.usra.edu/blog/2010/03/highway-mirage.html